It is often necessary to determine the amounts of different isotopes in a material. Isotopes are different forms of the same chemical element, having nuclei of different masses. For example, naturally occurring carbon consists predominantly of .sup.12 C, i.e., carbon having an atomic mass of 12 atomic mass units with small amounts of the .sup.13 C and .sup.14 C isotopes, having atomic masses of 13 and 14 AMU respectively. The .sup.12 C and .sup.13 C isotopes are stable, whereas the .sup.14 C isotope is radioactive, and spontaneously decays to other elements with time. In so-called "carbon dating", the ratio of .sup.14 C to .sup.12 C in a specimen is measured to obtain an indication of the age of the specimen. Numerous biological and chemical tests use radioactive tracers such as .sup.14 C. A carbon-containing compound which interacts with a biological or chemical system such as a living organism is prepared using .sup.14 C in place of naturally occurring carbon, so that the compound is "labelled" or "tagged" with the .sup.14 C. The biological specimen is then exposed to the labelled compound so that the specimen interacts with the labelled compound. This interaction produces a test specimen or analyte incorporating .sup.14 C from the labelled compound, in amounts directly related to the biological interaction of interest. For example, in radio immunoassay tests, the amounts of a particular antibody in a biological specimen can be measured by exposing the specimen to a .sup.14 C-labelled antigen adapted to bind chemically with the antibody. The amount of antigen taken up by the specimen, and hence the amount of .sup.14 C taken up by the specimen, provides a measure of the amount of antibody in the specimen. In other tests, the specimen may be a substance excreted or exuded by the biological specimen. For example, .sup.14 C-labelled urea may be administered to a living mammalian subject such as a human being. If certain bacteria are present in the subjects intestinal tract, the carbon dioxide exhaled in the subject's breath will include the labelling isotope incorporated in the urea. Thus, such bacteria can be detected by monitoring the ratio of .sup.14 C to .sup.12 C in the subject's breath.
.sup.14 C is ordinarily used as the labelling isotope in these and other tests because it can be detected by monitoring the radiation which it emits when it decays. Such monitoring may be performed using relatively simple instruments. However, the use of radioactive materials is undesirable. Such radioactive materials are inherently unstable. Moreover, although the amounts of radioactive materials used in tracer studies of this nature typically are small, any radioactivity is undesirable with respect to safety and health considerations. In theory, directly analogous tracer studies can be performed using the stable, rare isotope .sup.13 C as the labelling isotope instead of .sup.14 C. However, it is difficult to measure the amount of .sup.13 C or the ratio of .sup.13 C to .sup.12 C in a sample. Such measurements typically have been performed heretofore using mass spectrometers. The cost and complexity associated with mass spectrometry pose significant drawbacks. Moreover, mass spectrometry is unusable in certain situations. Mass spectrometry cannot distinguish between different chemical species having the same mass. Great care must be taken to eliminate background atoms, molecules and radicals having the same mass as the species of interest. Accordingly, there has been a long-felt need for improved methods of measuring the amounts of carbon isotopes in an analyte.
There have been corresponding needs for improved methods of measuring the amounts of isotopes of other elements in analytes. For example, a method of measuring the amount of the rare but stable oxygen isotope .sup.18 O, and/or the ratio of .sup.18 O to the common isotope .sup.16 O would be highly desirable. This need is particularly acute because ordinary water molecules (H.sub.2 O) have essentially the same mass. (18 AMU) as .sup.18 O atoms. It is ordinarily impractical to measure the .sup.18 O:.sup.16 O ratio of a sample containing even a trace amount of water by mass spectroscopy, particularly where the ratio .sup.18 O:.sup.16 O is small. The .sup.18 O signal is simply overwhelmed by the signal arising from water in the sample. Apparently for this reason, .sup.18 O has not been widely used as a tracer in chemical and biological studies. Similar needs exist with respect to other elements.
Various attempts have been made to determine the amounts of isotopes in samples by spectroscopic techniques, i.e., by measuring the response of the sample to applied radiant energy. It has long been known that the energy absorption spectrum of atoms of different isotopes differ from one another, and some work has been done towards exploiting these differences for monitoring the isotopic composition of an analyte. As set forth in Lee, High Resolution Infrared Diode Laser Spectroscopy for Isotope Analysis--Measurement of Isotopic Carbon monoxide, Applied Physics Letters, 48 (10), Mar. 10, 1986, pp. 619-621, a light beam from a tunable diode laser can be directed through a sample of carbon monoxide to a photodetector. The laser is tuned in succession to different wavelengths. Each such wavelength corresponds to a ground-state absorption wavelength of a carbon monoxide molecule containing a particular isotope of oxygen. The amount of light absorbed and hence the amount of light detected at each of these wavelengths is related to the amount of the particular oxygen isotope present in the carbon monoxide. This system, however, requires complex and highly sensitive instrumentation. The wavelengths absorbed by the different isotopic forms of CO are extremely close to one another, within the range of about 2119.581-2120.235 cm.sup.-1. To provide for precise tunability within this range, a so-called quantum well diode laser is employed. Such a laser must be operated at liquid nitrogen temperatures, and provides only a very weak signal, which in turn requires a large and complex liquid nitrogen cooled photo detector. Accordingly, this method has not been widely adopted.
Keller et al, Optogalvanic Spectroscopy in a Uranium Hollow Cathode Discharge, Opt. Soc. Am., Vol 69, No. 5, May 1979, pp. 738-742, discloses a spectroscopic method in which uranium metal is subjected to sputtering in a hollow cathode discharge. The discharge thus includes sputtered uranium atoms in the ground or unexcited state. This electrical discharge is subjected to irradiation by a laser at varying wavelengths. The interaction between the laser light and the discharge is monitored by monitoring the so-called optogalvanic effect, i.e., the change in the electrical impedance of the discharge upon irradiation. The optogalvanic effect produced by light at a so-called "hyperfine" absorption wavelength of .sup.238 U atoms is compared with the optogalvanic effect at a hyperfine absorption wavelength of .sup.235 U. This provides a measure of the isotopic ratio .sup.235 U/.sup.238 U. A generally similar approach is set forth in Gagne et al, Effet Optogalvanique Dans Une Decharge a Cathode Creuse: Mechanisme et Dosage Isotopique de l'araniun, Journal de Physique, C7, No. 11, Vol. 44, pp. C7-355 to C7-369 (November, 1983).
Another similar approach to the analysis of copper isotopes .sup.63 Cu and .sup.65 Cu is disclosed in Tong, New Laser Spectroscopic Technique for Stable-isotope Ratio Analysis, PhD. thesis, Iowa State University, Ames, Iowa December 1984, U.S. DOE report IS-T-1156. This thesis uses the optogalvanic effect to monitor hyperfine spectral components of optical absorption in an electrical discharge containing copper atoms. This approach requires a subsequent deconvolution step to obtain an estimate of the .sup.63 Cu and .sup.65 Cu components. Tong suggests that the technique could be used in conjunction with Cu-based tracer studies, as, for example, using a stable copper isotope as a tracer to study copper metabolism. A transition from a ground state of the copper atom is employed. The reference also states that it is "feasible" to observe the optogalvanic effect in transitions of the atoms originating from excited states as well as from ground states, but merely suggests that this allows one to choose "an appropriate excitation wavelength where there is minimal spectral interference." Attempts to monitor the hyperfine absorption of metal atoms, however, encounter serious drawbacks. The hyperfine spectra of the various isotopes include closely-spaced and overlapping absorption wavelengths so that complex equipment and mathematical deconvolution techniques are required to segregate the effects due to absorption by the different isotopes in the analyte.
A published summary of a grant application by Aerodyne Research, Inc., entitled "A Carbon-13 Isotope Analyzer", NSF Grant No. ISI 88-60778, Abstracts of Phase I Awards, NSF Small Business Innovation Research Program (SBIR) 1989, National Science Foundation, November 1989 describes a planned attempt to determine the .sup.13 C:.sup.12 C isotopic ratio of carbon monoxide by imaging the emission spectrum of a CO plasma and applying so-called "spectral processing algorithms" to suppress interference arising from various sources. This approach has not gained wide acceptance.
Thus, in spite all of this effort in the art heretofore, there have still been significant unmet needs for improved methods of isotopic analysis. The need for improved methods and apparatus applicable to relatively low atomic number elements such as carbon, nitrogen, oxygen and hydrogen has been particularly acute.